2021A&A...649A.122P

The frequency of Earth-sized planets in habitable zones appears to be higher around M-dwarfs, making these systems exciting laboratories to investigate planet formation. Observations of protoplanetary disks around very low-mass stars and brown dwarfs remain challenging and little is known about their properties. The disk around CIDA 1 (∼0.1-0.2M_☉) is one of the very few known disks that host a large cavity (20au radius in size) around a very low-mass star. We present new ALMA observations at Band 7 (0.9mm) and Band 4 (2.1mm) of CIDA 1 with a resolution of ∼0.05''x0.034''. These new ALMA observations reveal a very bright and unresolved inner disk, a shallow spectral index of the dust emission (∼2), and a complex morphology of a ring located at 20au. We also present X-shooter (VLT) observations that confirm the high accretion rate of CIDA 1 of M_acc=1.4x10^–8 M_☉/yr. This high value of M_acc, the observed inner disk, and the large cavity of 20 au exclude models of photo-evaporation to explain the observed cavity. When comparing these observations with models that combine planet-disk interaction, dust evolution, and radiative transfer, we exclude planets more massive than 0.5M_Jup as the potential origin of the large cavity because with these it is difficult to maintain a long-lived and bright inner disk. Even in this planet mass regime, an additional physical process may be needed to stop the particles from migrating inwards and to maintain a bright inner disk on timescales of millions of years. Such mechanisms include a trap formed by a very close-in extra planet or the inner edge of a dead zone. The low spectral index of the disk around CIDA 1 is difficult to explain and challenges our current dust evolution models, in particular processes like fragmentation, growth, and diffusion of particles inside pressure bumps.